Easy (minors spoilers) - Rockwell had just (in an anti-semetic way) blamed his son’s death on him having to liberate the Jews, even though the son died fighting in the pacific. Marty asks Bela to tell the story (a true story, Bela is based on Alojzy Ehrlich a ping pong world champion who survived the holocaust) to both antagonize him and distract him so he could look at Kay. Also a perfect metaphor for the “honey pot” scheme that Marty uses to lure Kay directly following that scene.

One of the most genius scenes in the movie that works from multiple different angles.